SSK Industries created the largest center-fire rifle ever, with a .905 caliber. Only 3 have been made. The lightest weighing in around 50 pounds. The ammunition, 2400 grain bullet, propelled by 240 grains of powder weighing in at half a pound each, and costing $40. The power is comparable to a World War I-era tank round or a 20mm cannon in terms of kinetic energy.
The JDJ was almost categorized as a "destructive device" but got the "sportsman exception" categorization instead. It is a beast. Watch the guys from Knight Rifles as they enjoy their unique experience.